1. Field of the Invention
The invention pertains to an array of balun driven dipole elements and arrays of such dipoles useful as a microwave radiating antenna.
2. Prior Art
Dipole antennas are well known in the prior art. A typical dipole antenna consists of dipole arms which are fed by balanced transmission lines or a balun connected to an unbalanced transmission line. In that latter case, the dipole is driven by an open-circuited unbalanced transmission line which is overlaid on the grounded antenna structure to form the balun and can either extend over the dipole in an "L" shape or be bent back towards the ground plane in a "J" shape. The operating frequency of a dipole antenna is determined by its geometric structure and is generally limited to a narrow bandwidth.
A typical example of a dipole antenna is disclosed in U.S. Pat. No. 3,845,490 to Manwarren et al. This reference discloses a stripline slotted balun dipole antenna, where a single "L" shaped driving transmission line is sandwiched between two dielectric sheets, each containing a balun dipole antenna. A "J" shaped microstrip transmission line is disclosed in U.S. Pat. No. 4,825,220 to Edward et al. This reference describes a planar balun dipole antenna and a structure that allows the geometry to be physically altered after fabrication to tune the antenna to a desired frequency. Edward also describes the use of a reflecting surface located perpendicular to the antenna to increase radiation efficiency in the direction tangent to the balun. In both these references, the disclosed antennas are optimized for a single frequency.
U.S. Pat. No. 3,239,838 to Kelleher discloses a dipole antenna mounted in an open-faced resonant cavity. This reference discloses a dipole antenna where the dipole arms are not placed at the termination points of the balun transmission lines, but rather, are placed near their ends, with the remaining part of the balun forming stubs. Additionally, the microstrip transmission line used to drive the antenna is not extended into the stub region. Further, Kelleher does not teach or suggest the use of these stubs to increase the bandwidth of the antenna.
Balun dipole antennas are particularly suited to fabrication in planar arrays. For example, U.S. Pat. No. 3,747,114 to Shyhalla illustrates a flat planar array of microwave radiating elements. The dipole elements are formed on a planar dielectric substrate. The transmission line distribution circuit which drives the antennas is also formed on a planar substrate. Shyhalla discloses circumscribing the entire antenna array within a protective frame to provide rigidity. However, no suggestion is made to circumscribe each dipole antenna with a ground plane extension.